The hypothetical protein CT813 is localized in the Chlamydia trachomatis inclusion membrane and is immunogenic in women urogenitally infected with C. trachomatis

The serodiagnositic value of Chlamydia trachomatis antigens in antibody detection using luciferase immunosorbent assay

Introduction

Among the different antigens used in the detection of anti-Chlamydia trachomatis antibodies, significant differences in sensitivity and specificity have been observed. Further evaluation of C. trachomatis antigens in antibody detection is urgently needed for the development and application of C. trachomatis serologic assays.

Methods

Chlamydia trachomatis antigens Pgp3, TmeA, InaC, and HSP60 were selected and used in luciferase immunosorbent assay (LISA). The detection results obtained from well-defined C. trachomatis positive and negative samples were compared with the commercial C. trachomatis ELISA (Mikrogen) for performance evaluation.

Results

Pgp3, TmeA, InaC, and HSP60-based LISA showed sensitivity of 92.8, 88.8, 90.4, and 94.4%, and specificity of 99.2, 99.2, 99.2, and 92%, respectively. ROC analysis indicated that Pgp3-based LISA showed similar performance to Mikrogen ELISA (AUC 0.986 vs. 0.993, p = 0.207). Furthermore, four C. trachomatis antigens achieved strong diagnostic efficiency, i.e., positive likelihood ratios [+LR] ≥ 10 in C. trachomatis-infected women and negative likelihood ratios [-LR] ≤ 0.1 in C. trachomatis negative low exposure risk children, but only Pgp3 and TmeA showed strong diagnostic value in general adults. In addition, Pgp3, TmeA, and InaC, but not HSP60, achieved high performance, i.e., both positive predictive value (PPV) and negative predictive value (NPV) ≥ 90.9%, and showed no significant cross-reactivity with anti-Chlamydiapneumoniae.

Conclusion

Three C. trachomatis species-specific antigens Pgp3, TmeA, and InaC show superior performance in the detection of anti-C. trachomatis antibody, indicating the potential to be used in developing C. trachomatis serologic tests.

The acetylase activity of Cdu1 regulates bacterial exit from infected cells by protecting Chlamydia effectors from degradation

Many cellular processes are regulated by ubiquitin-mediated proteasomal degradation. Pathogens can regulate eukaryotic proteolysis through the delivery of proteins with de-ubiquitinating (DUB) activities. The obligate intracellular pathogen Chlamydia trachomatis secretes Cdu1 (ChlaDUB1), a dual deubiquitinase and Lys-acetyltransferase, that promotes Golgi remodeling and survival of infected host cells presumably by regulating the ubiquitination of host and bacterial proteins. Here, we determined that Cdu1's acetylase but not its DUB activity is important to protect Cdu1 from ubiquitin-mediated degradation. We further identified three C. trachomatis proteins on the pathogen-containing vacuole (InaC, IpaM, and CTL0480) that required Cdu1's acetylase activity for protection from degradation and determined that Cdu1 and these Cdu1-protected proteins are required for optimal egress of Chlamydia from host cells. These findings highlight a non-canonical mechanism of pathogen-mediated protection of virulence factors from degradation after their delivery into host cells and the coordinated regulation of secreted effector proteins.

The acetylase activity of Cdu1 regulates bacterial exit from infected cells by protecting Chlamydia effectors from degradation

Many cellular processes are regulated by ubiquitin-mediated proteasomal degradation. Pathogens can regulate eukaryotic proteolysis through the delivery of proteins with de-ubiquitinating (DUB) activities. The obligate intracellular pathogen Chlamydia trachomatis secretes Cdu1 (ChlaDUB1), a dual deubiquitinase and Lys-acetyltransferase, that promotes Golgi remodeling and survival of infected host cells presumably by regulating the ubiquitination of host and bacterial proteins. Here we determined that Cdu1's acetylase but not its DUB activity is important to protect Cdu1 from ubiquitin-mediated degradation. We further identified three C. trachomatis proteins on the pathogen-containing vacuole (InaC, IpaM, and CTL0480) that required Cdu1's acetylase activity for protection from degradation and determined that Cdu1 and these Cdu1-protected proteins are required for optimal egress of Chlamydia from host cells. These findings highlight a non-canonical mechanism of pathogen-mediated protection of virulence factors from degradation after their delivery into host cells and the coordinated regulation of secreted effector proteins.

The emerging complexity of Chlamydia trachomatis interactions with host cells as revealed by molecular genetic approaches

Chlamydia trachomatis (Ct) is an intracellular bacterial pathogen that relies on the activity of secreted proteins known as effectors to promote replication and avoidance of immune clearance. Understanding the contribution of Ct effectors to pathogenesis has proven to be challenging, given that these proteins often perform multiple functions during intracellular infection. Recent advances in molecular genetic analysis of Ct have provided valuable insights into the multifaceted nature of secreted effector proteins and their impact on the interaction between Ct and host cells and tissues. This review highlights significant findings from genetic analysis of Ct effector functions, shedding light on their diverse roles. We also discuss the challenges faced in this field of study and explore potential opportunities for further research.

The bacterial effector GarD shields Chlamydia trachomatis inclusions from RNF213-mediated ubiquitylation and destruction

Chlamydia trachomatis is the leading cause of sexually transmitted bacterial infections and a major threat to women's reproductive health in particular. This obligate intracellular pathogen resides and replicates within a cellular compartment termed an inclusion, where it is sheltered by unknown mechanisms from gamma-interferon (IFNγ)-induced cell-autonomous host immunity. Through a genetic screen, we uncovered the Chlamydia inclusion membrane protein gamma resistance determinant (GarD) as a bacterial factor protecting inclusions from cell-autonomous immunity. In IFNγ-primed human cells, inclusions formed by garD loss-of-function mutants become decorated with linear ubiquitin and are eliminated. Leveraging cellular genome-wide association data, we identified the ubiquitin E3 ligase RNF213 as a candidate anti-Chlamydia protein. We demonstrate that IFNγ-inducible RNF213 facilitates the ubiquitylation and destruction of GarD-deficient inclusions. Furthermore, we show that GarD operates as a cis-acting stealth factor barring RNF213 from targeting inclusions, thus functionally defining GarD as an RNF213 antagonist essential for chlamydial growth during IFNγ-stimulated immunity.

Hypothetical protein FoDbp40 influences the growth and virulence of Fusarium oxysporum by regulating the expression of isocitrate lyase

Fungal growth is closely related to virulence. Finding the key genes and pathways that regulate growth can help elucidate the regulatory mechanisms of fungal growth and virulence in efforts to locate new drug targets. Fusarium oxysporum is an important plant pathogen and human opportunistic pathogen that has research value in agricultural and medicinal fields. A mutant of F. oxysporum with reduced growth was obtained by Agrobacterium tumefaciens-mediated transformation, the transferred DNA (T-DNA) interrupted gene in this mutant coded a hypothetical protein that we named FoDbp40. FoDbp40 has an unknown function, but we chose to explore its possible functions as it may play a role in fungal growth regulatory mechanisms. Results showed that F. oxysporum growth and virulence decreased after FoDbp40 deletion. FOXG_05529 (NCBI Gene ID, isocitrate lyase, ICL) was identified as a key gene that involved in the reduced growth of this mutant. Deletion of FoDbp40 results in a decrease of more than 80% in ICL expression and activity, succinate level, and energy level, plus a decrease in phosphorylated mammalian target of rapamycin level and an increase in phosphorylated 5′-adenosine monophosphate activated protein kinase level. In summary, our study found that the FoDbp40 regulates the expression of ICL at a transcriptional level and affects energy levels and downstream related pathways, thereby regulating the growth and virulence of F. oxysporum.

Computational Mining and Characterization of Hypothetical Proteins of Mycobacterium bovis Toward the Identification of Probable Vaccine Candidates

A hypothetical protein (HP) is one that is known to exist only on the basis of a corresponding gene but without any function assigned to it. Many HPs have emerged as attractive vaccine candidates against prokaryotic and eukaryotic pathogens as well as against cancers. Mycobacterium bovis is a serious veterinary pathogen of tremendous zoonotic importance. This protocol describes a computational workflow for the identification of the HPs of M. bovis with vaccine potential and their subsequent structural and functional characterization.

Got mutants? How advances in chlamydial genetics have furthered the study of effector proteins

Chlamydia trachomatis is the leading cause of infectious blindness and a sexually transmitted infection. All chlamydiae are obligate intracellular bacteria that replicate within a membrane-bound vacuole termed the inclusion. From the confines of the inclusion, the bacteria must interact with many host organelles to acquire key nutrients necessary for replication, all while promoting host cell viability and subverting host defense mechanisms. To achieve these feats, C. trachomatis delivers an arsenal of virulence factors into the eukaryotic cell via a type 3 secretion system (T3SS) that facilitates invasion, manipulation of host vesicular trafficking, subversion of host defense mechanisms and promotes bacteria egress at the conclusion of the developmental cycle. A subset of these proteins intercalate into the inclusion and are thus referred to as inclusion membrane proteins. Whereas others, referred to as conventional T3SS effectors, are released into the host cell where they localize to various eukaryotic organelles or remain in the cytosol. Here, we discuss the functions of T3SS effector proteins with a focus on how advances in chlamydial genetics have facilitated the identification and molecular characterization of these important factors.

Nafamostat mesylate inhibits chlamydial intracellular growth in cell culture and reduces chlamydial infection in the mouse genital tract

Urogenital Chlamydia trachomatis (C. trachomatis) infection is one of the most common bacterial sexually transmitted diseases worldwide. Untreated C. trachomatis infections that ascend to the upper genital tract lead to a series of severe complications. To search for novel antichlamydial drugs, we evaluated the effect of nafamostat mesylate (NM), a synthetic serine protease inhibitor, on chlamydial infection. NM inhibited chlamydial intracellular growth and reduced both the inclusion size and number in cell culture. NM may mainly target the intracellular reticulate bodies for inhibition. NM was also effective in enhancing chlamydial clearance from mouse genital tract when NM was applied to mice via intravaginal inoculation. The vaginal NM did not significantly alter inflammatory cytokine responses in the mouse genital tract. Thus, we have demonstrated a novel role of NM in inhibiting the obligate intracellular bacterium Chlamydia.

Chlamydia trachomatis Whole-Proteome Microarray Analysis of The Netherlands Chlamydia Cohort Study

Chlamydia trachomatis (Ct) whole-proteome microarrays were utilized to identify antibody patterns associated with infection; pelvic inflammatory disease (PID), tubal factor infertility, chronic pelvic pain (CPP) and ectopic pregnancy in a subsample of the Netherlands Chlamydia cohort study. Serum pools were analyzed on whole-proteome arrays. The 121 most reactive antigens identified during whole-proteome arrays were selected for further analysis with minimized microarrays that allowed for single sera analysis. From the 232 single sera; 145 (62.5%) serum samples were reactive for at least one antigen. To discriminate between positive and negative serum samples; we created a panel of in total 18 antigens which identified 96% of all microarray positive samples. Antigens CT_858; CT_813 and CT_142 were most reactive. Comparison of antibody reactivity's among women with and without Ct related sequelae revealed that the reactivity of CT_813 and CT_142 was less common among women with PID compared to women without (29.0% versus 58.6%, p = 0.005 and 25.8% versus 50.6%, p = 0.017 respectively). CT_858 was less common among CPP cases compared to controls (33.3% versus 58.6; p = 0.028). Using a whole-proteome array to select antigens for minimized arrays allows for the identification of novel informative antigens as general infection markers or disease associated antigens.

The multiple functions of the numerous Chlamydia trachomatis secreted proteins: the tip of the iceberg

Chlamydia trachomatis serovars are obligate intracellular bacterial pathogens mainly causing ocular and urogenital infections that affect millions of people worldwide and which can lead to blindness or sterility. They reside and multiply intracellularly within a membrane-bound vacuolar compartment, known as inclusion, and are characterized by a developmental cycle involving two morphologically and physiologically distinct chlamydial forms. Completion of the developmental cycle involves the secretion of > 70 C. trachomatis proteins that function in the host cell cytoplasm and nucleus, in the inclusion membrane and lumen, and in the extracellular milieu. These proteins can, for example, interfere with the host cell cytoskeleton, vesicular and non-vesicular transport, metabolism, and immune signalling. Generally, this promotes C. trachomatis invasion into, and escape from, host cells, the acquisition of nutrients by the chlamydiae, and evasion of cell-autonomous, humoral and cellular innate immunity. Here, we present an in-depth review on the current knowledge and outstanding questions about these C. trachomatis secreted proteins.

Structural and Functional Annotation of Conserved Virulent Hypothetical Proteins in Chlamydia Trachomatis: An In-Silico Approach

Background:

Though after a start of genome sequencing most of the protein sequences are deposited in databases, some proteins remain to be unannotated and functionally uncharacterized. Chlamydia trachomatis L2C is a gram-negative pathogen bacterium involved in causing severe disorders like lymphogranuloma venereum, nongonococcal urethritis, and cervicitis. <P> Objectives: Analyzing and annotating the hypothetical proteins can help to understand its pathogenicity and therapeutic hotspots. Its genome encodes a total of 221 hypothetical proteins and out of these, 14 hypothetical proteins are declared as virulent by virulence prediction server (VirulentPred). <P> Methods: In this study, the functional and structural analysis was carried out by conserve domain finding servers, protein function annotators and physiochemical properties predictors. Proteinprotein interactions studies revealed the involvement of these virulent HPs in a number of pathways, which would be of interest for drug designers. <P> Results: Classifier tool was used to classify the virulent hypothetical proteins into enzymes, membrane protein, transporter and regulatory protein groups. <P> Conclusion: Our study would help to understand the mechanisms of pathogenesis and new potential therapeutic targets for a couple of diseases caused by C. trachomatis.

Chlamydia trachomatis ct143 stimulates secretion of proinflammatory cytokines via activating the p38/MAPK signal pathway in THP-1 cells

Chlamydia trachomatis (Ct) infections can cause bacterial sexually-transmitted and preventable blindness. The Ct infections induced excessive cytokines generation which attributed to pathologic changes in host cells. However, the precise mechanisms of Ct-induced cytokines production are still unclear.CT143 protein was identified as a novel Ct specific protein with high immunogenicity. In the present study. The CT143 fusion protein was recombined and purified. The mice immune serum was prepared by immunizing BALB/c mice with the purified fusion protein. The specificity of the antibody was confirmed using Immunoblotting. Indirect immunoflurescence assay (IFA) and Immunoblotting assays were performed to detect the temporal and spatial characteristics of CT143 in Ct infected cells. ELISA was performed to analyze the secretion of proinflammatory cytokines IL-1β, IL-8 and TNF-α by human macrophages under the stimulation of CT143 protein. Finally, the involvement of p38 signaling in CT143-induced cytokine secretion was validated. CT143 protein was located in the inclusion body and represented an Elementary body (EB)-related protein, which may be encoded by the mid- and late-stage expressing genes. CT143 protein could stimulate the secretion of inflammatory cytokines in macrophages which differentiated from THP-1 This induction may be mediated by the activation of p38 signaling. In summary, CT143 protein is involved in inflammatory processes during Ct infection.

Immunoprofiling of Chlamydia trachomatis using whole-proteome microarrays generated by on-chip in situ expression

Using Chlamydia trachomatis (Ct) as a complex model organism, we describe a method to generate bacterial whole-proteome microarrays using cell-free, on-chip protein expression. Expression constructs were generated by two successive PCRs directly from bacterial genomic DNA. Bacterial proteins expressed on microarrays display antigenic epitopes, thereby providing an efficient method for immunoprofiling of patients and allowing de novo identification of disease-related serum antibodies. Through comparison of antibody reactivity patterns, we newly identified antigens recognized by known Ct-seropositive samples, and antigens reacting only with samples from cervical cancer (CxCa) patients. Large-scale validation experiments using high-throughput suspension bead array serology confirmed their significance as markers for either general Ct infection or CxCa, supporting an association of Ct infection with CxCa. In conclusion, we introduce a method for generation of fast and efficient proteome immunoassays which can be easily adapted for other microorganisms in all areas of infection research.

IL-6-mediated signaling pathways limit Chlamydia muridarum infection and exacerbate its pathogenicity in the mouse genital tract

Chlamydia muridarum induction of mouse hydrosalpinx, depending on both tubal infection and inflammation, has been used for investigating Chlamydia trachomatis pathogenesis. We now report that IL-6 both inhibits C. muridarum infection and exacerbates pathogenicity in the mouse genital tract. When intravaginally inoculated with a high dose of C. muridarum, IL-6-deficient mice developed more extensive genital tract infection with severe hydrosalpinx, suggesting that IL-6 is required for controlling the high dose infection but not essential for C. muridarum-induced pathology. However, at a low dose, IL-6-deficient mice still developed more extensive infection in the genital tract but no longer with significant pathology, suggesting that IL-6 is required for both controlling the low dose infection and exacerbating the low dose infection-induced pathology. The lack of hydrosalpinx in IL-6-deficient mice correlated with significantly reduced inflammatory infiltration in the oviduct tissue and decreased spleen CD4+ and CD8+ T cells that produce TNFα. Thus, IL-6-dependent pathways are important for both limiting chlamydial colonization in the genital tract mucosal tissues regardless of the infection doses and exacerbating chlamydial pathogenicity in the upper genital tract when IL-6-independent pathogenic mechanisms are not yet activated with a low infection dose.

The p47phox deficiency significantly attenuates the pathogenicity of Chlamydia muridarum in the mouse oviduct but not uterine tissues

The Chlamydia muridarum induction of the upper genital tract pathology in mice has been used to investigate the mechanisms of chlamydial pathogenesis. We report that the NCF1 (neutrophil cytosolic factor1)-encoded p47phox (phagocyte oxidase), an essential subunit of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase, contributes significantly to C. muridarum induction of hydrosalpinx. Mice lacking p47phox (p47phox-deficient) were no longer able to develop significant hydrosalpinx following an intravaginal infection with C. muridarum. However, there was no significant difference in uterine horn dilation (as a result of the endometrial glandular duct dilation) between the p47phox-deficient and -sufficient mice. Thus, the role of NADPH oxidase in chlamydial pathogenesis is restricted to the oviduct tissue rather than the entire upper genital tract. Interestingly, both the p47phox-deficient and -sufficient mice displayed similar levels of chlamydial live organism shedding from the lower genital tract, suggesting that the NADPH oxidase is not required for the mouse control of chlamydial infection in the lower genital tract. Furthermore, the p47phox deficiency did not affect the infectious organism burden in the upper genital tract tissues, indicating that the NADPH-oxidase activity is not necessary for the mouse prevention of chlamydial ascension from the lower to upper genital tracts. However, the p47phox-defieicnt mice displayed a significantly reduced chronic inflammatory infiltration in the oviduct but not uterine tissues, supporting the finding that the NADPH oxidase activity is required for chlamydial induction of dilation in the oviduct but not the endometrial glandular duct. Thus, we have demonstrated a significant role of the host NADPH oxidase in promoting chronic inflammatory pathology in the oviduct following chlamydial infection.

Localization and characterization of two putative TMH family proteins in Chlamydia psittaci

Chlamydia psittaci (C. psittaci), an obligate intracellular agent of psittacosis, causes an atypical pneumonia in humans. The transmembrane head proteins (TMH) of C. psittaci, putatively belong to the Inc family and presumably play similar roles. CPSIT_0844 and CPSIT_0846 were the putative TMH proteins of C. psittaci. To identify these two proteins, antisera were raised with fusion proteins which were prokaryotic expressed in Escherichia coli and purified. By immunofluorescence assay, CPSIT_0844 and CPSIT_0846 were localized in the inclusion membrane of C. psittaci-infected cells. By RT-PCR and western blot analysis to detect the temporal expression, CPSIT_0844 and CPSIT_0846 were detected as early as 12h post-infection (p.i.) and 6h p.i., separately; meanwhile, in secretions monitored with immunofluorescence assay, these proteins were observed in the inclusion membrane at 18h p.i. and remained in the inclusion membrane throughout the growth cycle. CPSIT_0844 and CPSIT_0846 could specifically be recognized by the antiserum of C. psittaci but failed to react with the antiserums of Chlamydia trachomatis and Chlamydia pneumoniae, which is consistent with the fact that they had no significant orthologs in C. trachomatis and C. pneumoniae. These results revealed that CPSIT_0844 and CPSIT_0846, the putative TMH family proteins, might be unique to C. psittaci and could be used to diagnose the infection caused by C. psittaci. Moreover, CPSIT_0844 and CPSIT_0846 could induce the expression of the inflammatory cytokines IL-1β, IL-6 and TNF-α in THP-1 cells, which might contribute to chlamydia-induced inflammatory pathologies.

Ubiquitin systems mark pathogen-containing vacuoles as targets for host defense by guanylate binding proteins

Many microbes create and maintain pathogen-containing vacuoles (PVs) as an intracellular niche permissive for microbial growth and survival. The destruction of PVs by IFNγ-inducible guanylate binding protein (GBP) and immunity-related GTPase (IRG) host proteins is central to a successful immune response directed against numerous PV-resident pathogens. However, the mechanism by which IRGs and GBPs cooperatively detect and destroy PVs is unclear. We find that host cell priming with IFNγ prompts IRG-dependent association of Toxoplasma- and Chlamydia-containing vacuoles with ubiquitin through regulated translocation of the E3 ubiquitin ligase tumor necrosis factor (TNF) receptor associated factor 6 (TRAF6). This initial ubiquitin labeling elicits p62-mediated escort and deposition of GBPs to PVs, thereby conferring cell-autonomous immunity. Hypervirulent strains of Toxoplasma gondii evade this process via specific rhoptry protein kinases that inhibit IRG function, resulting in blockage of downstream PV ubiquitination and GBP delivery. Our results define a ubiquitin-centered mechanism by which host cells deliver GBPs to PVs and explain how hypervirulent parasites evade GBP-mediated immunity.

The Chlamydia pneumoniae Inclusion Membrane Protein Cpn1027 Interacts with Host Cell Wnt Signaling Pathway Regulator Cytoplasmic Activation/Proliferation-Associated Protein 2 (Caprin2)

We previously identified hypothetical protein Cpn1027 as a novel inclusion membrane protein that is unique to Chlamydia pneumoniae. In the current study, using a yeast-two hybrid screen assay, we identified host cell cytoplasmic activation/proliferation-associated protein 2 (Caprin2) as an interacting partner of Cpn1027. The interaction was confirmed and mapped to the C-termini of both Cpn1027 and Caprin2 using co-immunoprecipitation and GST pull-down assays. A RFP-Caprin2 fusion protein was recruited to the chlamydial inclusion and so was the endogenous GSK3β, a critical component of the β-catenin destruction complex in the Wnt signaling pathway. Cpn1027 also co-precipitated GSK3β. Caprin2 is a key regulator of the Wnt signaling pathway by promoting the recruitment of the β-catenin destruction complex to the cytoplasmic membrane in the presence of Wnt signaling while GSK3β is required for priming β-catenin for degradation in the absence of Wnt signaling. The Cpn1027 interactions with Caprin2 and GSK3β may allow C. pneumoniae to actively sequester the β-catenin destruction complex so that β-catenin is maintained even in the absence of extracellular Wnt activation signals. The maintained β-catenin can trans-activate Wnt target genes including Bcl-2, which may contribute to the chlamydial antiapoptotic activity. We found that the C. pneumoniae-infected cells were more resistant to apoptosis induction and the anti-apoptotic activity was dependent on β-catenin. Thus, the current study suggests that the chlamydial inclusion protein Cpn1027 may be able to manipulate host Wnt signaling pathway for enhancing the chlamydial anti-apoptotic activity.

Integrating chemical mutagenesis and whole-genome sequencing as a platform for forward and reverse genetic analysis of Chlamydia

Gene inactivation by transposon insertion or allelic exchange is a powerful approach to probe gene function. Unfortunately, many microbes, including Chlamydia, are not amenable to routine molecular genetic manipulations. Here we describe an arrayed library of chemically induced mutants of the genetically intransigent pathogen Chlamydia trachomatis, in which all mutations have been identified by whole-genome sequencing, providing a platform for reverse genetic applications. An analysis of possible loss-of-function mutations in the collection uncovered plasticity in the central metabolic properties of this obligate intracellular pathogen. We also describe the use of the library in a forward genetic screen that identified InaC as a bacterial factor that binds host ARF and 14-3-3 proteins and modulates F-actin assembly and Golgi redistribution around the pathogenic vacuole. This work provides a robust platform for reverse and forward genetic approaches in Chlamydia and should serve as a valuable resource to the community.

Characterization of CPAF critical residues and secretion during Chlamydia trachomatis infection

CPAF (chlamydial protease-like activity factor), a Chlamydia serine protease, is activated via proximity-induced intermolecular dimerization that triggers processing and removal of an inhibitory peptide occupying the CPAF substrate-binding groove. An active CPAF is a homodimer of two identical intramolecular heterodimers, each consisting of 29-kDa N-terminal and 35-kDa C-terminal fragments. However, critical residues for CPAF intermolecular dimerization, catalytic activity, and processing were defined in cell-free systems. Complementation of a CPAF-deficient chlamydial organism with a plasmid-encoded CPAF has enabled us to characterize CPAF during infection. The transformants expressing CPAF mutated at intermolecular dimerization, catalytic, or cleavage residues still produced active CPAF, although at a lower efficiency, indicating that CPAF can tolerate more mutations inside Chlamydia-infected cells than in cell-free systems. Only by simultaneously mutating both intermolecular dimerization and catalytic residues was CPAF activation completely blocked during infection, both indicating the importance of the critical residues identified in the cell-free systems and exploring the limit of CPAF's tolerance for mutations in the intracellular environment. We further found that active CPAF was always detected in the host cell cytoplasm while nonactive CPAF was restricted to within the chlamydial inclusions, regardless of how the infected cell samples were treated. Thus, CPAF translocation into the host cell cytoplasm correlates with CPAF enzymatic activity and is not altered by sample treatment conditions. These observations have provided new evidence for CPAF activation and translocation, which should encourage continued investigation of CPAF in chlamydial pathogenesis.

Complement factor C5 but not C3 contributes significantly to hydrosalpinx development in mice infected with Chlamydia muridarum

Hydrosalpinx is a pathological hallmark of tubal infertility associated with chlamydial infection. However, the mechanisms of hydrosalpinx remain unknown. Here, we report that complement factor 5 (C5) contributes significantly to chlamydial induction of hydrosalpinx. Mice lacking C5 (C5(-/-)) failed to develop any hydrosalpinx, while ∼42% of the corresponding wild-type mice (C5(+/+)) did so following intravaginal infection with Chlamydia muridarum. Surprisingly, deficiency in C3 (C3(-/-)), an upstream component of the complement system, did not affect mouse susceptibility to chlamydial induction of hydrosalpinx. Interestingly, C5 activation was induced by chlamydial infection in oviducts of C3(-/-) mice, explaining why the C3(-/-) mice remained susceptible to chlamydial induction of hydrosalpinx. Similar levels of live chlamydial organisms were recovered from oviduct tissues of both C5(-/-) and C5(+/+) mice, suggesting that C5 deficiency did not affect C. muridarum ascending infection. Furthermore, C5(-/-) mice were still more resistant to hydrosalpinx induction than C5(+/+) mice, even when live C. muridarum organisms were directly delivered into the upper genital tract, both confirming the role of C5 in promoting hydrosalpinx and indicating that the C5-facilitated hydrosalpinx was not due to enhancement of ascending infection. The C5(-/-) mice displayed significantly reduced lumenal inflammatory infiltration and cytokine production in oviduct tissue, suggesting that C5 may contribute to chlamydial induction of hydrosalpinx by enhancing inflammatory responses.

Chlamydial induction of hydrosalpinx in 11 strains of mice reveals multiple host mechanisms for preventing upper genital tract pathology

The female lower genital tract is constantly exposed to microbial infection, some of which can ascend to and cause pathology such as hydrosalpinx in the upper genital tract, which can affect fertility. To understand host mechanisms for preventing upper genital tract pathology, we screened 11 inbred strains of mice for hydrosalpinx induction by C. muridarum. When examined on days 60 to 80 after intravaginal infection, the 11 strains fell into 3 groups based on their hydrosalpinx severity: CBA/J and SJL/J mice were highly susceptible with a hydrosalpinx score of 5 or greater; Balb/c, C57BL/6J, C57BL/10J, C3H/HeJ and C3H/HeN were susceptible with a score between 1 and <5; NOD/ShiLtJ, DBA/1J, DBA/2J and A/J were resistant with a score of <1. The diverse range of mouse susceptibility to hydrosalpinx induction may reflect the varied clinical outcomes of C. trachomatis-infected women. When the 11 strains were infected via an intrauterine inoculation to bypass the requirement for ascension, higher incidence and more severe hydrosalpinges were induced in most mice, indicating that the interaction between chlamydial ascension and host control of ascension is critical for determining susceptibility to hydrosalpinx development in many mice. However, a few mouse strains resisted significant exacerbation of hydrosalpinx by intrauterine infection, indicating that these mice have evolved ascension-independent mechanisms for preventing upper genital tract pathology. Together, the above observations have demonstrated that different strains of mice can prevent upper genital tract pathology by using different mechanisms.

Lack of long-lasting hydrosalpinx in A/J mice correlates with rapid but transient chlamydial ascension and neutrophil recruitment in the oviduct following intravaginal inoculation with Chlamydia muridarum

Lower genital tract infection with Chlamydia trachomatis and C. muridarum can induce long-lasting hydrosalpinx in the upper genital tract of women and female mice, respectively. However, A/J mice were highly resistant to induction of long-lasting hydrosalpinx by C. muridarum. We further compared host inflammatory responses and chlamydial infection courses between the hydrosalpinx-resistant A/J mice and CBA/J mice known to be susceptible to hydrosalpinx induction. Both mouse strains developed robust pyosalpinx during the acute phase followed by hydrosalpinx during the chronic phase. However, the hydrosalpinges disappeared in A/J mice by day 60 after infection, suggesting that some early hydrosalpinges are reversible. Although the overall inflammatory responses were indistinguishable between CBA/J and A/J mice, we found significantly more neutrophils in oviduct lumen of A/J mice on days 7 and 10, which correlated with a rapid but transient oviduct invasion by C. muridarum with a peak infection on day 7. In contrast, CBA/J mice developed a delayed and extensive oviduct infection. These comparisons have revealed an important role of the interactions of oviduct infection with inflammatory responses in chlamydial induction of long-lasting hydrosalpinx, suggesting that a rapid but transient invasion of oviduct by chlamydial organisms can prevent the development of the long-lasting hydrosalpinges.

Signaling via tumor necrosis factor receptor 1 but not Toll-like receptor 2 contributes significantly to hydrosalpinx development following Chlamydia muridarum infection

Chlamydial infection in the lower genital tract can lead to hydrosalpinx, which is accompanied by activation of both pattern recognition receptor TLR2- and inflammatory cytokine receptor TNFR1-mediated signaling pathways. In the current study, we compared the relative contributions of these two receptors to chlamydial induction of hydrosalpinx in mice. We found that mice with or without deficiencies in TLR2 or TNFR1 displayed similar time courses of live organism shedding from vaginal swabs, suggesting that these receptor-mediated signaling pathways are not required for controlling chlamydial lower genital infection. However, mice deficient in TNFR1 but not TLR2 developed significantly reduced hydrosalpinx. The decreased pathogenicity correlated with a significant reduction in interleukin-17 by in vitro-restimulated splenocytes of TNFR1-deficient mice. Although TLR2-deficient mice developed hydrosalpinx as severe as that of wild-type mice, peritoneal macrophages from mice deficient in TLR2 but not TNFR1 produced significantly reduced cytokines upon chlamydial stimulation, suggesting that reduced macrophage responses to chlamydial infection do not always lead to a reduction in hydrosalpinx. Thus, we have demonstrated that the signaling pathways triggered by the cytokine receptor TNFR1 play a more significant role in chlamydial induction of hydrosalpinx than those mediated by the pattern recognition receptor TLR2, which has laid a foundation for further revealing the chlamydial pathogenic mechanisms.

Reduced live organism recovery and lack of hydrosalpinx in mice infected with plasmid-free Chlamydia muridarum

Plasmid-free Chlamydia trachomatis and Chlamydia muridarum fail to induce severe pathology. To evaluate whether the attenuated pathogenicity is due to insufficient infection or inability of the plasmidless chlamydial organisms to trigger pathological responses, we compared plasmid-competent and plasmid-free C. muridarum infections in 5 different strains of mice. All 5 strains developed hydrosalpinx following intravaginal inoculation with plasmid-competent, but not inoculation with plasmid-free, C. muridarum. The lack of hydrosalpinx induction by plasmid-free C. muridarum correlated with significantly reduced live organism recovery from the lower genital tract and shortened infection in the upper genital tract. The plasmid-free C. muridarum organisms failed to induce hydrosalpinx even when the organisms were directly inoculated into the oviduct via an intrabursal injection, which was accompanied by significantly reduced survival of the plasmidless organisms in the genital tracts. Furthermore, plasmid-competent C. muridarum organisms after UV inactivation were no longer able to induce hydrosalpinx even when directly delivered into the oviduct at a high dose. Together, these observations suggest that decreased survival of and shortened infection with plasmid-free C. muridarum may contribute significantly to its attenuated pathogenicity. We conclude that adequate live chlamydial infection in the oviduct may be necessary to induce hydrosalpinx.

Induction of protective immunity against Chlamydia muridarum intracervical infection in DBA/1j mice

We previously reported that intracervical inoculation with Chlamydia muridarum induced hydrosalpinx in DBA/1j mice, but intravaginal inoculation failed to do so. In the current study, we found unexpectedly that intrabursal inoculation of live chlamydial organisms via the oviduct failed to induce significant hydrosalpinx. We further tested whether primary infection via intravaginal or intrabursal inoculation could induce protective immunity against hydrosalpinx following intracervical challenge infection. Mice infected intravaginally with C. muridarum were fully protected from developing hydrosalpinx, while intrabursal inoculation offered partial protection. We then compared immune responses induced by the two genital tract inoculations. Both inoculations induced high IFNγ and IL-17 T cell responses although the ratio of IgG2a versus IgG1 in intravaginally infected mice was significantly higher than in mice infected intrabursally. When the antigen-specificities of antibody responses were compared, both groups of mice dominantly recognized 24 C. muridarum antigens, while each group preferentially recognized unique sets of antigens. Thus, we have demonstrated that intrabursal inoculation is neither effective for causing hydrosalpinx nor efficient in inducing protective immunity in DBA/1j mice. Intravaginal immunization, in combination with intracervical challenge infection in DBA/1j mice, can be a useful model for understanding mechanisms of chlamydial pathogenicity and protective immunity.

Oviduct infection and hydrosalpinx in DBA1/j mice is induced by intracervical but not intravaginal inoculation with Chlamydia muridarum

Intravaginal infection with C. muridarum in mice often results in hydrosalpinx similar to that found in women urogenitally infected with C. trachomatis, making the C. muridarum lower genital tract infection murine model suitable for studying C. trachomatis pathogenesis. To our surprise, DBA1/j mice were highly resistant to hydrosalpinx following an intravaginal infection with C. muridarum although these mice were as susceptible to lower genital tract infection as other mouse strains. A significantly lower level of C. muridarum organisms was recovered from the oviduct of DBA1/j mice, correlating the resistance to hydrosalpinx with reduced ascension of C. muridarum to the oviduct. The DBA1/j resistance to hydrosalpinx was effectively overcome by intracervical inoculation with C. muridarum. The intracervically inoculated DBA1/j mice developed severe hydrosalpinx with the highest levels of live C. muridarum organisms recovered from uterine tissue on day 3 and oviduct tissue on day 7 post inoculation while in intravaginally inoculated DBA1/j mice, the peak of live organism recovery from uterine tissue was delayed to day 7 with no rise in the amount of live organisms recovered from the oviduct. These observations have not only validated the correlation between hydrosalpinx and live organism invasion in the oviduct but also demonstrated that the intracervical inoculation, by promoting rapid chlamydial replication in the uterine epithelial cells and ascension to the oviduct of DBA1/j mice, may be used for further understanding chlamydial pathogenic mechanisms. The above findings also suggest that strategies aimed at reducing tubal infection may be most effective in blocking tubal pathology.

Chlamydia trachomatis GlgA is secreted into host cell cytoplasm

Glycogen has been localized both inside and outside Chlamydia trachomatis organisms. We now report that C. trachomatis glycogen synthase (GlgA) was detected in both chlamydial organism-associated and -free forms. The organism-free GlgA molecules were localized both in the lumen of chlamydial inclusions and in the cytosol of host cells. The cytosolic GlgA displayed a distribution pattern similar to that of a known C. trachomatis-secreted protease, CPAF. The detection of GlgA was specific since the anti-GlgA antibody labeling was only removed by preabsorption with GlgA but not CPAF fusion proteins. GlgA was detectable at 12h and its localization into host cell cytosol only became apparent at 24h after infection. The cytosolic localization of GlgA was conserved among all C. trachomatis serovars. However, the significance of the GlgA secretion into host cell cytoplasm remains unclear since, while expression of chlamydial GlgA in HeLa cells increased glycogen stores, it did not affect a subsequent infection with C. trachomatis. Similar to several other C. trachomatis-secreted proteins, GlgA is immunogenic in women urogenitally infected with C. trachomatis, suggesting that GlgA is expressed and may be secreted into host cell cytosol during C. trachomatis infection in humans. These findings have provided important information for further understanding C. trachomatis pathogenic mechanisms.

Contribution of interleukin-12 p35 (IL-12p35) and IL-12p40 to protective immunity and pathology in mice infected with Chlamydia muridarum

The p35 molecule is unique to interleukin-12 (IL-12), while p40 is shared by both IL-12 and IL-23. IL-12 promotes Th1 T cell responses, while IL-23 promotes Th17 T cell responses. The roles of IL-12p35- and IL-12p40-mediated responses in chlamydial infection were compared in mice following an intravaginal infection with Chlamydia muridarum. Mice deficient in either IL-12p35 or p40 both developed similar but prolonged infection time courses, confirming the roles of IL-12-mediated immune responses in clearing primary infection. However, all mice, regardless of genotype, cleared reinfection within 2 weeks, suggesting that an IL-12- or IL-23-independent adaptive immunity is protective against chlamydial infection. All infected mice developed severe oviduct hydrosalpinx despite the increased Th2 responses in IL-12p35- or IL-12p40-deficient mice, suggesting that Th2-dominant responses can contribute to Chlamydia-induced inflammatory pathology. Compared to IL-12p35 knockout mice, the IL-12p40-deficient mice exhibited more extensive spreading of chlamydial organisms into kidney tissues, leading to significantly increased incidence of pyelonephritis, which both confirms the role of IL-12 or IL-23-independent host responses in Chlamydia-induced pathologies and suggests that in the absence of IL-12/IFN-γ-mediated Th1 immunity, an IL-23-mediated response may play an important role in restricting chlamydial organisms from spreading into distal organs. These observations together provide important information for both understanding chlamydial pathogenesis and developing anti-Chlamydia vaccines.

Identification of a novel nuclear localization signal sequence in Chlamydia trachomatis-secreted hypothetical protein CT311

We previously reported that Chlamydia trachomatis hypothetical protein CT311 was secreted out of chlamydial inclusion and into host cell cytosol. We now found that CT311 further entered host cell nucleus at the late stage of infection and continued to accumulate in the nucleus of C. trachomatis-infected cells. When CT311 was expressed via a transgene in mammalian cells, CT311 protein was exclusively detected in the nucleus, suggesting that CT311 by itself is sufficient for nuclear targeting. However, preexisting nuclear CT311 did not affect subsequent chlamydial infection. Using deletion constructs, we mapped a nuclear localization signal sequence of CT311 to residues 21 to 63 (²¹AVEGKPLSRAAQLRERRKDLHVSGKPSPRYALKKRALEAKKNK⁶³). This sequence was sufficient for targeting a heterologous protein into mammalian cell nucleus and it contains two independent clusters of basic residues (³⁴RERRK³⁸ and ⁵³KKRALEAKKNK⁶³ respectively). Deletion or alanine substitution of the basic residues in either cluster led to loss of nuclear targeting activity, suggesting that both clusters are critical for the nuclear targeting function. These observations have demonstrated that the hypothetical protein CT311 possesses a novel nuclear localization signal sequence with dual modules of basic residues for targeting host cell nucleus during Chlamydia trachomatis infection.

Role for chlamydial inclusion membrane proteins in inclusion membrane structure and biogenesis

The chlamydial inclusion membrane is extensively modified by the insertion of type III secreted effector proteins. These inclusion membrane proteins (Incs) are exposed to the cytosol and share a common structural feature of a long, bi-lobed hydrophobic domain but little or no primary amino acid sequence similarity. Based upon secondary structural predictions, over 50 putative inclusion membrane proteins have been identified in Chlamydia trachomatis. Only a limited number of biological functions have been defined and these are not shared between chlamydial species. Here we have ectopically expressed several C. trachomatis Incs in HeLa cells and find that they induce the formation of morphologically distinct membranous vesicular compartments. Formation of these vesicles requires the bi-lobed hydrophobic domain as a minimum. No markers for various cellular organelles were observed in association with these vesicles. Lipid probes were incorporated by the Inc-induced vesicles although the lipids incorporated were dependent upon the specific Inc expressed. Co-expression of Inc pairs indicated that some colocalized in the same vesicle, others partially overlapped, and others did not associate at all. Overall, it appears that Incs may have an intrinsic ability to induce membrane formation and that individual Incs can induce membranous structures with unique properties.

PORF5 plasmid protein of Chlamydia trachomatis induces MAPK-mediated pro-inflammatory cytokines via TLR2 activation in THP-1 cells

Infection with Chlamydia trachomatis induces inflammatory pathologies in the urogenital tract that can lead to infertility and ectopic pregnancy. Pathogenesis of infection has been mostly attributed to excessive cytokine production. However, precise mechanisms on how C. trachomatis triggers this production, and which protein(s) stimulate inflammatory cytokines remains unknown. In the present study, the C. trachomatis pORF5 protein induced tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and interleukin-8 (IL-8) in dose- and time-dependent manners in the THP-1 human monocyte cell line. We found that intracellular p38/mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK)/MAPK signaling pathways were required for the induction of TNF-α, IL-1β and IL-8. Blockade of toll-like receptor 2 (TLR2) signaling reduced induction levels of TNF-α, IL-8 and IL-1β. We concluded that the C. trachomatis pORF5 protein might contribute to the inflammatory processes associated with chlamydial infections.

Immunogenicity of Cpn0425 and its localization in cells infected with Chlamydophila pneumoniae

The present study aimed to determine the intracellular localization of Cpn0425 in Chlamydophila pneumoniae-infected cells. The recombinant plasmid pGEX-6p/Cpn0425 was transformed into E.coli Bl21 cells to express the fusion protein. Following purification with glutathione S-transferase (GST) resin chromatography, the Cpn0425 fusion protein was used to induce immunity in mice to develop monoclonal and polyclonal antibodies, which were subsequently used to localize the endogenous Cpn0425 protein by indirect immunofluorescence assay (IFA). ELISA was used to determine the immunogenicity of the Cpn0425 plasmid protein by recognizing the pool sera of patients infected with Chlamydia trachomatis and the pool sera of mice immunized with the Cpn0425 fusion protein. The Cpn0425 gene was expressed as the GST-Cpn0425 fusion protein in E. coli and its antibody was prepared by immunizing mice with the fusion protein. An anti-GST-Cpn0425 antibody was used to localize the protein in cells infected with Chlamydophila pneumoniae AR-39 using an IFA. The anti-GST-CT058 antibody detected an inclusion signal in the IFA. Cpn0425 protein strongly reacted with antiserum. Although Cpn0425 protein is not a secreted protein, it has good immunogenicity. Therefore, this protein may be useful for developing vaccines against Chlamydophila pneumoniae infection.

Chlamydia trachomatis antigens recognized in women with tubal factor infertility, normal fertility, and acute infection

OBJECTIVE

To identify Chlamydia trachomatis antigens associated with tubal factor infertility and acute infection.

METHODS

A C trachomatis proteome array was used to compare antibody profiles among women with tubal factor infertility, normal fertility, and acute C trachomatis infection.

RESULTS

Thirteen immunodominant antigens reacted with 50% or more sera from all women (n=73). Six C trachomatis antigens were uniquely recognized in women with tubal factor infertility. Combining fragmentation of the six antigens with serum sample dilution, chlamydial antigens HSP60, CT376, CT557, and CT443 could discriminate between women with tubal factor infertility and women with normal fertility with a sensitivity of 63% (95% confidence interval [CI] 0.41-0.77) and specificity of 100% (95% CI 0.91-1), respectively. These antigens were designated as tubal factor infertility-associated antigens. However, these tubal factor antigens were unable to distinguish tubal factor infertility patients from those with acute infection. A combination of CT875 and CT147 distinguished women with acute infection from all other C trachomatis-exposed women with a detection sensitivity of 63% (95% CI 0.41-0.77) and specificity of 100% (95% CI 0.95-1), respectively. Thus, CT875 and CT147 were designated as acute infection-associated antigens.

CONCLUSION

A sequential screening of antibodies against panels of C trachomatis antigens can be used to identify women with tubal factor infertility and acute C trachomatis infection.

LEVEL OF EVIDENCE

II.

Localization of Chlamydia trachomatis hypothetical protein CT311 in host cell cytoplasm

The chlamydia-specific hypothetical protein CT311 was detected both inside and outside of the chlamydial inclusions in Chlamydia trachomatis-infected cells. The extra-inclusion CT311 molecules were distributed in the host cell cytoplasm with a pattern similar to that of CPAF, a known Chlamydia-secreted protease. The detection of CT311 was specific since the anti-CT311 antibody labeling was only removed by absorption with CT311 but not CPAF fusion proteins. In addition, both anti-CT311 and anti-CPAF antibodies only detected their corresponding endogenous proteins without cross-reacting with each other or any other antigens in the whole cell lysates of C. trachomatis-infected cells. Although both CT311 and CPAF proteins were first detected 12 h after infection, localization of CT311 into host cell cytosol was delayed until 24 h while CPAF secretion into host cell cytosol was already obvious by 18 h after infection. The host cell cytosolic localization of CT311 was further confirmed in human primary cells. CT311 was predicted to contain an N-terminal secretion signal sequence and the CT311 signal sequence directed secretion of PhoA into bacterial periplasmic region in a heterologous assay system, suggesting that a sec-dependent pathway may play a role in the secretion of CT311 into host cell cytosol. This hypothesis is further supported by the observation that secretion of CT311 in Chlamydia-infected cells was blocked by a C16 compound known to inhibit signal peptidase I. These findings have provided important molecular information for further understanding the C. trachomatis pathogenic mechanisms.

Chlamydia trachomatis secretion of an immunodominant hypothetical protein (CT795) into host cell cytoplasm

The Chlamydia-specific hypothetical protein CT795 was dominantly recognized by human antisera produced during C. trachomatis infection but not by animal antisera raised against dead chlamydia organisms. The immundominant region recognized by the human antibodies was mapped to the N-terminal fragment T22-S69. The endogenous CT795 was detected in the cytoplasm of host cells during C. trachomatis infection and was highly enriched in the host cytosolic fraction but absent in the purified chlamydia organisms, suggesting that CT795 is synthesized and secreted into host cell cytoplasm without incorporation into the organisms. All C. trachomatis serovars tested secreted CT795. A predicted signal peptide of CT795 directed the mature PhoA to cross Escherichia coli inner membranes. The secretion of CT795 in Chlamydia-infected cells was inhibited by a C(16) compound targeting signal peptidase I, but not by a C(1) compound known to block the type III secretion pathway. These results suggest that CT795, like CPAF (a Chlamydia-secreted virulence factor), is secreted into the host cell cytoplasm via a sec-dependent mechanism and not by a type III secretion pathway. The above characterizations of CT795 have provided important information for further understanding the potential roles of CT795 in C. trachomatis pathogenesis.

A Chlamydia trachomatis OmcB C-terminal fragment is released into the host cell cytoplasm and is immunogenic in humans

The Chlamydia trachomatis outer membrane complex protein B (OmcB) is an antigen with diagnostic and vaccine relevance. To further characterize OmcB, we generated antibodies against OmcB C-terminal (OmcBc) and N-terminal (OmcBn) fragments. Surprisingly, the anti-OmcBc antibody detected dominant signals in the host cell cytosol, while the anti-OmcBn antibody exclusively labeled intrainclusion signals in C. trachomatis-infected cells permeabilized with saponin. Western blot analyses revealed that OmcB was partially processed into OmcBc and OmcBn fragments. The processed OmcBc was released into host cell cytosol, while the OmcBn and remaining full-length OmcB were retained within the chlamydial inclusions. The organism-associated OmcB epitopes became detectable only after the C. trachomatis-infected cells were permeabilized with strong detergents such as SDS. However, the harsh permeabilization conditions also led to the leakage of the already secreted OmcBc and chlamydia-secreted protease (CPAF) out of the host cells. The OmcBc processing and release occurred in all biovars of C. trachomatis. Moreover, the released OmcBc but not the retained OmcBn was highly immunogenic in C. trachomatis-infected women, which is consistent with the concept that exposure of chlamydial proteins to host cell cytosol is accompanied by increased immunogenicity. These observations have provided important information for further exploring/optimizing OmcB as a target for the development of diagnosis methods and vaccines.

Multi-genome identification and characterization of chlamydiae-specific type III secretion substrates: the Inc proteins

Background

Chlamydiae are obligate intracellular bacteria that multiply in a vacuolar compartment, the inclusion. Several chlamydial proteins containing a bilobal hydrophobic domain are translocated by a type III secretion (TTS) mechanism into the inclusion membrane. They form the family of Inc proteins, which is specific to this phylum. Based on their localization, Inc proteins likely play important roles in the interactions between the microbe and the host. In this paper we sought to identify and analyze, using bioinformatics tools, all putative Inc proteins in published chlamydial genomes, including an environmental species.

Results

Inc proteins contain at least one bilobal hydrophobic domain made of two transmembrane helices separated by a loop of less than 30 amino acids. Using bioinformatics tools we identified 537 putative Inc proteins across seven chlamydial proteomes. The amino-terminal segment of the putative Inc proteins was recognized as a functional TTS signal in 90% of the C. trachomatis and C. pneumoniae sequences tested, validating the data obtained in silico. We identified a macro domain in several putative Inc proteins, and observed that Inc proteins are enriched in segments predicted to form coiled coils. A surprisingly large proportion of the putative Inc proteins are not constitutively translocated to the inclusion membrane in culture conditions.

Conclusions

The Inc proteins represent 7 to 10% of each proteome and show a great degree of sequence diversity between species. The abundance of segments with a high probability for coiled coil conformation in Inc proteins support the hypothesis that they interact with host proteins. While the large majority of Inc proteins possess a functional TTS signal, less than half may be constitutively translocated to the inclusion surface in some species. This suggests the novel finding that translocation of Inc proteins may be regulated by as-yet undetermined mechanisms.

Chlamydia trachomatis secretion of hypothetical protein CT622 into host cell cytoplasm via a secretion pathway that can be inhibited by the type III secretion system inhibitor compound 1

Using antibodies raised with C. trachomatis fusion proteins, we localized a hypothetical protein encoded by the ORF ct622 in the cytoplasm of C. trachomatis-infected mammalian cells. The detection was specific since the antibody labelling of CT622 protein was removed by preabsorption with CT622 but not other fusion proteins. We similarly confirmed that CT621, a known secretion protein encoded by a hypothetical ORF downstream of ct622, was secreted into host cell cytosol. Proteins CT622 and CT621 displayed a similar secretion pattern, with both intra-inclusion and host cell cytosol localization, that was distinct from that of CPAF (chlamydial protease/proteasome-like activity factor). However, the expression and secretion kinetics differed significantly between CT622 and CT621: CT622 mRNA was detected at 2 h, protein at 6 h and secretion of protein into host cell cytoplasm at 36 h post-infection, while CT621 mRNA was detected at 8 h, protein at 16 h and secretion at 24 h. The secretion of both CT622 and CT621 was blocked by N'-(3,5-dibromo-2-hydroxybenzylidene)-4-nitrobenzohydrazide (compound 1), an inhibitor known to target the type III secretion system of bacteria. These results suggest that CT621 and CT622 may fulfil different functions during chlamydial intracellular growth. Further characterization of these proteins may generate important information for understanding chlamydial pathogenesis.

Mice deficient in MyD88 Develop a Th2-dominant response and severe pathology in the upper genital tract following Chlamydia muridarum infection

MyD88, a key adaptor molecule required for many innate immunity receptor-activated signaling pathways, was evaluated in a Chlamydia muridarum urogenital tract infection model. Compared with wild-type mice, MyD88 knockout (KO) mice failed to produce significant levels of inflammatory cytokines in the genital tract during the first week of chlamydial infection. MyD88 KO mice developed a Th2-dominant whereas wild-type mice developed a Th1/Th17-dominant immune response after chlamydial infection. Despite the insufficient production of early inflammatory cytokines and lack of Th1/Th17-dominant adaptive immunity, MyD88 KO mice appeared to be as resistant to chlamydial intravaginal infection as wild-type mice based on the number of live organisms recovered from vaginal samples. However, significantly high numbers of chlamydial organisms were detected in the upper genital tract tissues of MyD88 KO mice. Consequently, MyD88 KO mice developed more severe pathology in the upper genital tract. These results together have demonstrated that MyD88-dependent signaling pathway is not only required for inflammatory cytokine production in the early phase of host response to chlamydial infection but also plays a critical role in the development of Th1/Th17 adaptive immunity, both of which may be essential for limiting ascending infection and reducing pathology of the upper genital tract by chlamydial organisms.

Chlamydia vaccine candidates and tools for chlamydial antigen discovery

The failure of the inactivated Chlamydia-based vaccine trials in the 1960s has led researchers studying Chlamydia to take cautious and rational approaches to develop safe and effective chlamydial vaccines. Subsequent research efforts focused on three areas. The first is the analysis of the immunobiology of chlamydial infection in animal models, with supporting clinical studies, to identify the immune correlates of both protective immunity and pathological responses. Second, recent radical improvements in genomics, proteomics and associated technologies have assisted in the implementation of creative approaches to search for suitable vaccine candidates. Third, progress in the analysis of host response and adjuvanticity regulating both innate and adaptive immunity at the mucosal site of infection has led to progress in the design of optimal delivery and adjuvant systems for enhancing protective immunity. Considerable progress has been made in the first two areas but research efforts to better define the factors that regulate immunity at mucosal sites of infection and to develop strategies to boost protective immunity via immunomodulation, effective delivery systems and potent adjuvants, have remained elusive. In this article, we will summarize progress in these areas with a focus on chlamydial vaccine antigen discovery, and discuss future directions towards the development of a safe and effective chlamydial vaccine.

Distinct roles of CD28- and CD40 ligand-mediated costimulation in the development of protective immunity and pathology during Chlamydia muridarum urogenital infection in mice

Infection with Chlamydia muridarum in the mouse urogenital tract can induce both protective immunity and inflammatory pathologies, which has been used as a model for understanding the immune and pathogenic mechanisms of C. trachomatis infection. We compared the roles of CD28- and CD40 ligand (CD40L)-mediated costimulation in C. muridarum infection. Mice with CD28 or CD80/CD86 gene knockout (KO) displayed an infection course similar to that of wild-type mice during both primary and secondary infection, suggesting that CD28-mediated costimulation is not required for protection against C. muridarum infection. However, mice deficient in CD40L or CD40 displayed a prolonged infection course after primary or secondary infection, suggesting that CD40-CD40L costimulation plays an essential role in the development of anti-C. muridarum immunity. Interestingly, the CD28- or CD80/CD86-deficient mice displayed significantly lower levels of inflammatory pathologies in the upper genital tracts after primary infection, although the attenuation in inflammation was no longer significant during secondary infection. However, the CD40L or CD40 KO mice developed inflammatory pathologies as severe as those in wild-type mice following either primary or secondary infection despite the obvious deficits in adaptive immunity in these KO mice. The resistance of CD28 or CD80/CD86 KO mice to chlamydial infection correlated with production of gamma interferon, while the development of inflammatory pathologies in CD40L or CD40 KO mice correlated with the production of other proinflammatory cytokines in mouse urogenital tracts during the early stages of the infection. These observations together suggest that C. muridarum-induced protective immunity and inflammatory pathologies can be mediated by distinct costimulatory signals.

Antibodies from women urogenitally infected with C. trachomatis predominantly recognized the plasmid protein pgp3 in a conformation-dependent manner

Background

C. trachomatis organisms carry a cryptic plasmid that encodes 8 open reading frames designated as pORF1 to 8. It is not clear whether all 8 pORFs are expressed during C. trachomatis infection in humans and information on the functionality of the plasmid proteins is also very limited.

Results

When antibodies from women urogenitally infected with C. trachomatis were reacted with the plasmid proteins, all 8 pORFs were positively recognized by one or more human antibody samples with the recognition of pORF5 protein (known as pgp3) by most antibodies and with the highest titers. The antibody recognition of the pORFs was blocked by C. trachomatis-infected HeLa but not normal HeLa cell lysates. The pgp3 fusion protein-purified human IgG detected the endogenous pgp3 in the cytosol of C. trachomatis-infected cells with an intracellular distribution pattern similar to that of CPAF, a chlamydial genome-encoded protease factor. However, the human antibodies no longer recognized pgp3 but maintained recognition of CPAF when both antigens were linearized or heat-denatured. The pgp3 conformation is likely maintained by the C-terminal 75% amino acid sequence since further deletion blocked the binding by the human antibodies and two conformation-dependent mouse monoclonal antibodies.

Conclusion

The plasmid-encoded 8 proteins are both expressed and immunogenic with pgp3 as the most immunodominant antigen during chlamydial infection in humans. More importantly, the human anti-pgp3 antibodies are highly conformation-dependent. These observations have provided important information for further understanding the function of the plasmid-encoded proteins and exploring the utility of pgp3 in chlamydial diagnosis and vaccination.

The chlamydial plasmid-encoded protein pgp3 is secreted into the cytosol of Chlamydia-infected cells

The chlamydial cryptic plasmid encodes eight putative open reading frames (ORFs), designated pORF1 to -8. Antibodies raised against these ORF proteins were used to localize the endogenous proteins during chlamydial infection. We found that the pORF5 protein (also known as pgp3) was detected mainly in the cytosol of Chlamydia-infected cells, while the remaining seven proteins were found inside the chlamydial inclusions only. The pgp3 distribution pattern in the host cell cytosol is similar to but not overlapping with that of chlamydial protease/proteasome-like activity factor (CPAF), a chlamydial genome-encoded protein known to be secreted from chlamydial inclusions into the host cell cytosol. The anti-pgp3 labeling was removed by preabsorption with pgp3 but not CPAF fusion proteins and vice versa, demonstrating that pgp3 is a unique secretion protein. This conclusion is further supported by the observation that pgp3 was highly enriched in cytosolic fractions and had a minimal presence in the inclusion-containing nuclear fractions prepared from Chlamydia-infected cells. The pgp3 protein was detected as early as 12 h after infection and was secreted by all chlamydial species that carry the cryptic plasmid, suggesting that there is a selection pressure for maintaining pgp3 secretion during chlamydial infection. Although expression of pgp3 in the host cell cytosol via a transgene did not alter the susceptibility of the transfected cells to the subsequent chlamydial infection, purified pgp3 protein stimulated macrophages to release inflammatory cytokines, suggesting that pgp3 may contribute to chlamydial pathogenesis.

Characterization of fifty putative inclusion membrane proteins encoded in the Chlamydia trachomatis genome

Although the Chlamydia trachomatis genome is predicted to encode 50 inclusion membrane proteins, only 18 have been experimentally localized in the inclusion membrane of C. trachomatis-infected cells. Using fusion proteins and anti-fusion protein antibodies, we have systematically evaluated all 50 putative inclusion membrane proteins for their localization in the infected cells, distribution patterns, and effects on subsequent chlamydial infection when expressed ectopically, as well as their immunogenicity during chlamydial infection in humans. Twenty-two of the 50 proteins were localized in the inclusion membrane, and 7 were detected inside the inclusions, while the location of the remaining 21 was not defined. Four (CT225, CT228, CT358, and CT440) of the 22 inclusion membrane-localized proteins were visualized in the inclusion membrane of Chlamydia-infected cells for the first time in the current study. The seven intra-inclusion-localized proteins were confirmed to be chlamydial organism proteins in a Western blot assay. Further characterization of the 50 proteins revealed that neither colocalization with host cell endoplasmic reticulum nor inhibition of subsequent chlamydial infection by ectopically expressed proteins correlated with the inclusion membrane localization. Interestingly, antibodies from women with C. trachomatis urogenital infection preferentially recognized proteins localized in the inclusion membrane, and the immunodominant regions were further mapped to the region predicted to be on the cytoplasmic side of the inclusion membrane. These observations suggest that most of the inclusion membrane-localized proteins are both expressed and immunogenic during C. trachomatis infection in humans and that the cytoplasmic exposure may enhance the immunogenicity.

SNARE protein mimicry by an intracellular bacterium

Many intracellular pathogens rely on host cell membrane compartments for their survival. The strategies they have developed to subvert intracellular trafficking are often unknown, and SNARE proteins, which are essential for membrane fusion, are possible targets. The obligate intracellular bacteria Chlamydia replicate within an intracellular vacuole, termed an inclusion. A large family of bacterial proteins is inserted in the inclusion membrane, and the role of these inclusion proteins is mostly unknown. Here we identify SNARE-like motifs in the inclusion protein IncA, which are conserved among most Chlamydia species. We show that IncA can bind directly to several host SNARE proteins. A subset of SNAREs is specifically recruited to the immediate vicinity of the inclusion membrane, and their accumulation is reduced around inclusions that lack IncA, demonstrating that IncA plays a predominant role in SNARE recruitment. However, interaction with the SNARE machinery is probably not restricted to IncA as at least another inclusion protein shows similarities with SNARE motifs and can interact with SNAREs. We modelled IncA's association with host SNAREs. The analysis of intermolecular contacts showed that the IncA SNARE-like motif can make specific interactions with host SNARE motifs similar to those found in a bona fide SNARE complex. Moreover, point mutations in the central layer of IncA SNARE-like motifs resulted in the loss of binding to host SNAREs. Altogether, our data demonstrate for the first time mimicry of the SNARE motif by a bacterium.

Molecular mechanisms of host-pathogen interactions and their potential for the discovery of new drug targets

Vaccines and chemotherapy have undeniably been the discoveries in the field of biomedical research that have exerted the biggest impact on the improvement of public health. Nevertheless, the development of bacterial resistance to antibiotics has co-evolved over time with the discovery of new drugs. This entails the necessity for continuous research on new anti-infectious agents. The current review highlights recent discoveries in the molecular mechanisms of specific host pathogen interactions and their potential for drug discovery. The focus is on facultative and obligate intracellular pathogens (Mycobacterium, Chlamydia and Legionella) and their manipulation of host cells in regard to inhibition of phagosome maturation and cell death. Furthermore, the composition and role of the SecA2 and the ESX-1 secretion pathways in bacterial virulence and manipulation of infected host cells is discussed. The central hypothesis proposed in this review is that the characterization of bacterial proteins and lipids involved in host cell manipulation (modulins) will provide an abundance of new drug targets. One advantage of targeting such bacterial modulins for drug development is that these anti-modulin drugs will not disrupt the beneficial host microflora and therefore have fewer side effects.